General purpose adhesives generally undergo shrinkage when curing. As a result of shrinkage large stresses are built up in the adhesive matrix. Thus, when shrinkable adhesives are used to join similar or dissimilar materials, particularly in combination with a reinforcing material (which has a high modulus) which cannot undergo shrinkage, large stresses are built up in the adhesive matrix portion. These stresses can only be relieved by failure of the adhesive matrix, and the matrix then pulls away from the reinforcing material or it undergoes cohesive failure producing void formation or microcracking.
An object of the invention is to provide a general purpose adhesive which undergoes expansion on cure in contrast to the usual adhesive which undergoes shrinkage under the same usage conditions.
Another object of this invention is to provide a spiroortho ester as a comonomer for the epoxy resin which is used in the manufacture of these general purpose adhesives.
A further object of this invention is to provide a general purpose adhesive formula so that the resulting bond that is produced between the adherents are bonded much more securely to achieve an adhesive matrix that is substantially void and stress-free.